1. Field of the Invention
The present invention relates to a uninterruptible power-supply system, and specifically to a uninterruptible power-supply system suitable for rectifying a three-phase AC by a converter including a power semiconductor module, converting the power into AC power of which the voltage and frequency can be changed by an inverter, and reducing a charging voltage of the storage battery or boosting a discharging voltage of the same.
2. Description of the Related Art
An uninterruptible power-supply system (referred to as UPS below) is an apparatus for using a power conversion apparatus to stably supply the power without an interruption of the power supply to a load when a commercial power supply and the like which is a stationary power supply has an abnormality. As IT use has been innovated in recent years, a demand of the UPS in a data center and the like has increased. Since the UPS for the data center is laid near cities where the land price is high, it is desired to reduce an installation area, that is, to miniaturize the apparatus.
The UPS is often used by using the storage battery as a voltage source when the commercial power supply and the like has an abnormality. The UPS discharges the storage battery and supplies the power to an inverter. Then, the UPS converts the power into a desired voltage and frequency and supplies it to a load. Conventionally, a chopper has a role for reducing a DC voltage rectified by the converter to charge the storage battery. The chopper also has a role for increasing a low-pressure storage battery voltage and supplying it to the inverter. The chopper is operated as a so-called bidirectional chopper (referred to as charge/discharge chopper below).
Generally, in the above-mentioned charge/discharge chopper circuit, an output power at the time of charge is small. However, it is necessary to have a rated power equivalent to that of the inverter at the time of discharge. Therefore, a switching element included in the charge/discharge chopper preferably has a rated current and rated loss capable of bearing a load at the time of discharge. Accordingly, when the commercial power supply is normally operated, a power semiconductor module included in the charge/discharge chopper and a reactor in a previous stage of the charge/discharge chopper have an excessive performance. The excessive performance increases the size of a charge/discharge circuit.
As a solution to a problem that the size of the charge/discharge circuit increases, for example, a system has been known in which an input connection destination of the converter is switched to the commercial power supply or the storage battery as described in JP 4951476 A. Specifically, at the time of commercial power supply abnormality, a rated DC voltage is output to the inverter by supplying the discharging power of the storage battery to the converter and the charge/discharge chopper which have performed a boosting operation. The structure described in JP 4951476 A can distribute the discharging power to the converter and the charge/discharge chopper. Therefore, miniaturization of the charge/discharge chopper itself and the reactor in the previous stage of the charge/discharge chopper can be realized.